Of the challenges that face Parkinson's disease (PD) research, few are more pressing than early detection of preclinical disease. Currently no method exists to diagnose PD before the irreversible deterioration of the brain. The lack of useful biomarkers is also a roadblock for studies of disease-modifying therapies, leading to the strong consensus that a major initiative to develop PD biomarkers is essential. Sense of smell is among the first casualties of PD, with olfactory loss occurring in up to 90% of patients. Furthermore, an association between impaired olfaction and subsequent PD diagnosis has been found in prospective population-based studies, suggesting that olfactory dysfunction is a very early sign of idiopathic PD. Although interest in using olfaction as a biomarker for PD has increased substantially in recent years, the mechanism of olfactory loss and its cause-and-effect relationship to ?-synucleinopathy, a major pathological hallmark of PD, remain unclear. This knowledge gap has created a major roadblock to diagnostic and therapeutic progress. Thus, it is critical that we understand the olfactory system neuroanatomical changes that lead to loss of smell in both the preclinical and symptomatic phases of PD, since an understanding of this pathway in well-defined animal models would significantly impact our knowledge of the mechanisms for loss of dopaminergic neurons, and at the same time provide a means for rapid and objective assessment of disease risk. Our over-arching hypothesis is that olfactory damage in PD is an early event based on specific neuroanatomical changes with measurable functional impact, and is directly linked to upstream ?-synuclein toxicity. We will assess the olfactory system by capitalizing on a well-established murine genetic model of PD, namely ?-synuclein aggregate deposition in mThy1-hSNCA transgenic mice, and emphasizing experiments during the prodromal stage. This model is highly relevant to the etiology of olfactory dysfunction in human PD owing to its construct and face validity with PD in humans. Moreover, we will achieve high-level definition of olfactory system injury by exploiting: i) tagged odorant receptor-expressing transgenic mice (M72- or P2-IRES- tau-LacZ) for precise tracking of olfactory sensory neurons (OSNs) and their stereotypic olfactory sensory map, ii) thy1-yellow fluorescent protein (YFP) transgenic mice for analyzing mitral cells, their neural circuitry and archicortical projections, and iii) advanced non-invasive functional neuroimaging (fMRI, MEMRI, and GT-tMRI). By combining these incisive modalities with behavioral, biochemical, and histological assessments, we will elucidate the neuroanatomical substrate for early olfactory loss in PD, while also addressing the role of synucleinopathy in PD-associated olfactory dysfunction. Three aims are proposed: Aim 1 - To understand the role of post-synaptic ?-synuclein toxicity in structural and functional changes in the olfactory sensory neurons and their stereotypic olfactory sensory map leading to impaired sense of smell in PD. Aim 2 - To elucidate how abnormal ?-synuclein aggregates induce structural changes in the olfactory bulb mitral cells, their neural circuitry and modulatory interneurons, leading to impaired sense of smell in PD. Aim 3 - To elucidate how abnormal ?-synuclein aggregates alter archicortical projections and targets of mitral cells leading to impaire odor identification and discrimination in PD. These studies are significant, as the results will enhance understanding of the mechanisms of olfactory loss in PD and its relationship to pathologic ?-synuclein. Our results will in turn provide clues to new therapies, as well as biomarkers useful for early detection, prognostication, and monitoring of either disease progression or outcomes of disease-modifying therapies. Our approaches are innovative, since they exploit incisive genetic and cutting-edge analytic tools, and the study is highly feasible due to the team's expertise in olfaction, PD, neurodegeneration, oxidative stress, neurobehavioral testing, and advanced imaging technologies.